Mixing PV panels in series with MPPT

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Bunkysdad
Bunkysdad Registered Users Posts: 38 ✭✭
in General Solar Power Topics #1
Unfortunately, while in a camp ground my neighbor backed over one of my three matched 120w PV panels…Problem is I cannot find an exact wattage and VOC replacement…My understanding is that when in a series config current (not matched VOC) matters, and that the system will be governed by the panel with the lowest current…

Having said that, my sense is that when adding another albeit somewhat unmatched panel, that in series the voltage is still additive which will produce an overall net gain, with the mixed panel deficit partially overcome due to the MPPT…

Also, prior to this malady, I was measuring 57v with the three 120w panels in series but only 5 amps per Victron MPPT solar App …However, this calculated at about 260watts of ‘real world’ harvest (I’m happy with that :).  )….But my sense is that these are ‘pre-MPPT’ amps rather than the likely MPPT increased amps going to the battery?

My continued brainstorming has not priduced an answer to either (ugg!!)…

Thanks 

Bunkysdad
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  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,046 ✭✭✭✭✭
    edited July 2023 #2
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    Can you give stats for the new panel?
    You are correct regarding your observed numbers. The 260 watts going into the controller should come out very near 260 watts, only at battery voltage and increased amps .i;e; 12.8 volts @ 20.3 amps

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

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  • NANOcontrol
    NANOcontrol Registered Users Posts: 261 ✭✭✭
    #3
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    Having everything match is the short answer.  Every panel matching within 10% is the practical solution for economy. Nothing wrong with replacing panel with a higher current one is fine.  In a permanent installation that has occasional shading at one edge, doubling up a single panel in the shade is an effective solution.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,457 admin
    #4
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    The simple equation for MPPT controllers (assuming that the input/output votlages/currents are within specifications of hardware, neglecting 5% or so controller losses, and ~81% of Name Plate rated wattage deratings--i.e., 100 Watt STD rating for panel * 0.81 = 81 Watts daily best case typical maximum)...
    • Power = Voltage * Current
    • 5 amps * 57 volts (series array) = 285 Watts (array) (actual array measured current/voltage)
    • 285 Watts / 14.75 volts battery charging = 19.33 Amps battery charging current (note Amps varies with actual battery voltage)
    For series panels in an array (MPPT controller), don't worry about Voc/Vmp rating of new panel... Just "match" the Imp (within 5% to 10%). Just make sure the Voc/Vmp of the array with "new panel" does not exceed the solar charge controller Vpanel-max input voltage.

    With a higher (or lower) Vmp rated 3rd panel, the MPPT controller will "adjust the array Vmp voltage" to match the "new array" actual output.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Bunkysdad
    Bunkysdad Registered Users Posts: 38 ✭✭
    #5
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    Many thanks for some very good info!!

    Bunkysdad
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  • Bunkysdad
    Bunkysdad Registered Users Posts: 38 ✭✭
    #6
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    littleharbor2 said:
    Can you give stats for the new panel?
    You are correct regarding your observed numbers. The 260 watts going into the controller should come out very near 260 watts, only at battery voltage and increased amps .i;e; 12.8 volts @ 20.3 amps

    ***************

    For the proposed new replacement panel:

    VOC 22.2
    ISC 7.14
    VMP 18.49
    IMP 6.49
    120w


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  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,046 ✭✭✭✭✭
    #7
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    Bunkysdad said:
    littleharbor2 said:
    Can you give stats for the new panel?
    You are correct regarding your observed numbers. The 260 watts going into the controller should come out very near 260 watts, only at battery voltage and increased amps .i;e; 12.8 volts @ 20.3 amps

    ***************

    For the proposed new replacement panel:

    VOC 22.2
    ISC 7.14
    VMP 18.49
    IMP 6.49
    120w


    . What is your Max Voc. of the charge controller? You might want/need to make 2 strings of two panels to stay at safe voltage levels. With a 12 volt systemyou'll still get high enough voltage.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

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  • Bunkysdad
    Bunkysdad Registered Users Posts: 38 ✭✭
    #8
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    Oops, BTW here are the stats on the remaining two matched panels:

    VOC = 25.35v

    Vmp = 21.45v

    Imp = 5.59a


    Victron MPPT controller is 100v 30a


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  • littleharbor2
    littleharbor2 Solar Expert Posts: 2,046 ✭✭✭✭✭
    #9
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     Not the best match. The new panels are lower current which will cause each string to have nearly one amp lower current. The voltage mismatch will detract from the total output as well.

     Go ahead and wire them 2s2p and see what you get going to your batteries. I'm not sure if it would make any difference whether you series wire the matching panels together in each string or whether you should wire one of each different panels together into  strings. Might be worth trying each way and test output.

     Do you have a DC clamp meter? That would be the easiest way to find out what you're producing. Check the actual current going into the battery multiplied by the battery voltage at that time.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric,  460 Ah. 24 volt LiFePo4 battery bank. Plenty of Baja Sea of Cortez sunshine.

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  • Bunkysdad
    Bunkysdad Registered Users Posts: 38 ✭✭
    edited July 2023 #10
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    I’m still confused (dummy is me!!… ) as the proposed new panel is at 6.49 Imp while the two remaining matched panels are at 5.59 Imp, so about .9 amps greater than the existing panels…So my sense is that once connected in series, the current of 6.49a would likely drop to near that of the two existing panels - does this scenario sound reasonable ??

    I’m kinda lost in the woods on this - can’t thank you guys enough :smile:

    Bunkysdad
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,457 admin
    #11
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    Yes, you are correct... Approximately 1 amp less, then P=V*I or 17.5 volts * 1 amp = 17.5 watts less harvest than that panel is capable of... (I.e., 120 Watt panel will produce around 100 Watts in this configuration).

    Other than a bit less power from that replacement panel, there should be no other issues (that I can think of).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • Bunkysdad
    Bunkysdad Registered Users Posts: 38 ✭✭
    edited July 2023 #12
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    Thanks, got it :) , so in absence of procuring an exact replacement, do you think that this path could be an acceptable compromise??…I would think that the MPPT aspect (e.g. added voltage in series config) will still provide a gain over just the two panels..

    Bunkysdad
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  • Bunkysdad
    Bunkysdad Registered Users Posts: 38 ✭✭
    edited July 2023 #13
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    Well Good News, I was able to find the panel by using a different search criteria :), delivery not expected for 1-2 mos…

    https://www.amazon.com/dp/B09X4LH154/ref=emc_b_5_t?th=1

    I really appreciate all the kindly and informative tutelage - I will most definitely keep this all in mind!!

    Also I like the clamp-on meter suggestion, the two panel string idea and the math formula - so glad to have a happy ending!

    Bunkysdad
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